The present invention relates to blow molding, pressure forming, and more particularly to apparatus for blow molding and pressure forming parts with smooth surfaces within a short cycle time.
Blow molding is a plastic forming process which produces hollow parts. The process involves the extrusion of a molten tube of resin called a parison, into a mold. The mold closes around the parison, pinching the bottom of the parison closed, air or another gas is introduced causing the tube to expand against the cool surfaces of the mold. When the parison comes in contact with the cool surface of the mold, the plastic at the surface quickly freezes. This creates surface imperfections such as die lines, fold lines, pores and voids.
One important requisite for the use of blow molded parts for exterior applications such as automotive body panels or doors, is that the surface be smooth and blemish free. The parts should be as smooth as parts currently made from sheet metal. In order to use conventionally made blow molded parts for such applications, labor intensive sanding and polishing operations are necessary.
An approach to improve the surface quality of blow molded parts is to thermally cycle a low thermal mass mold using hot and cold oils which flow in oil passages in the mold. This approach requires complex process control and adds an additional minute to cycle time. Large capital costs are involved for the equipment including a complex mold structure. Pressure forming is the forming of plastic sheets into parts through the use of vacuum on one side of the heated sheet and atmospheric or greater air pressure on the other side of the heated sheet. Contact between a heated sheet and a cool mold causes surface porosity or voids and portions of the mold are not completely filled with resin.
It is an object of the present invention to provide apparatus to blow mold parts on hot mold surfaces in a short cycle time.
It is another object of the present invention to provide apparatus to blow mold parts with smooth surfaces which can use existing equipment without major modifications.